Digital data memory technology plays an important role in the information industry. In general, memory is divided into two types: volatile and non-volatile. Volatile memory requires power to maintain the stored information and is therefore suitable as primary temporary storage, whereas non-volatile memory can stably retain the stored information even when the memory is not powered and is suitable for use as secondary more permanent storage. Volatile memory includes conventional random-access memory (RAM)—static RAM (SRAM), and dynamic RAM (DRAM). Non-volatile memory includes ferromagnetic RAM (FeRAM), magneto-resistive RAM (MRAM), and flash memory.
Flash memory is a non-volatile memory, the contents of which may be electrically erased and rewritten. It is widely used to provide robust data storage in small devices, for example in memory cards and USB flash drives for general storage and transfer of data. As storage and write-and-erase speed requirements increase and the size of devices decreases, limitations in flash memory performance call for a replacement technology.
Phase-change memory (PCM) is a possible replacement technology. Like flash memory, phase-change memory, also referred to as phase-change random access memory (PRAM), is non-volatile; once switched, i.e. programmed, it remains stable in that state until it is switched again.
Phase-change memory exploits changes in the physical state of a phase-change material of the phase-change memory. The data storage mechanism of phase-change memory, as the name suggests, depends on a reversible and detectable phase change in the phase-change material—for example a change from an amorphous state to a crystalline state. In an amorphous state, the arrangement of the constituent atoms in the material exhibits no long-range order. In contrast, in a crystalline state the constituent atoms are arranged in an orderly repeating pattern.
Because each physical state has measurable distinct properties, for example distinct electrical properties such as resistivity or distinct optical properties such as index of refraction, phase-change material may be used in PCM cells to store bits of data. A PCM element in the amorphous state, characterized by its high resistivity state, may represent a logic “0” data bit value whereas a PCM element in the crystalline state, characterized by its low resistivity state, may represent a logic “1” data bit value. The phase-change memory can be switched between the amorphous state and the crystalline state reliably through heat. For example, intense heat of short duration is used to melt the phase-change material in a given spot. When the intense heat is stopped the temperature drops so quickly that the atoms freeze in an amorphous state before they can arrange themselves in a crystalline state. To switch back to the crystalline state, less-intense heat of longer duration is used to heat the amorphous area of the material without melting thereby allowing the atoms to rearrange themselves into a crystalline state. To read the recorded, programmed information, a probe may be used to measure the electrical resistivity of the area of material. The high resistivity measurement of the amorphous state is read as a binary “0” whereas the low resistivity measurement of the crystalline state is read as a binary “1”.
With PCM, data may be rewritten, re-programmed, without the need of a separate erase step and can exhibit write rates comparable to SRAM and DRAM. Novel faster computer memory architecture that eliminates the use of multiple tiers of system memory may also be possible with PCM.
To date, PCM technology uses chalcogenide-based phase-change material. A chalcogenide is a bronze alloy, an alloy that contains an element from the oxygen/sulphur family (old Group IVA, new Group 16) of the Periodic Table. The chalcogenide-based PCM material most commonly used is an alloy of germanium (Ge), antimony (Sb) and tellurium (Te), Ge2Sb2Te5 referred to as GST. Chalcogenide-based PCM materials generally exhibit a two to three orders of magnitude difference in resistivity between the amorphous state and the crystalline state. Unfortunately, this large difference in resistivity between the two states requires a correspondingly large electric voltage, or power, to enable the phase change and switch between the two states. Moreover, the elements antimony (Sb) and tellurium (Te) are known to be toxic and the manufacture of chalcogenide-based phase-change material requires special handling and disposal of these elements.
There is therefore a need for PCM that is non-volatile, requires low power, and provides high capacity memory with fast programmable rates.